Gas plasma generation is widely used in a variety of integrated circuit (IC) fabrication processes, including plasma etching, plasma enhanced chemical vapor deposition (PECVD), and plasma sputter deposition applications. Generally, plasmas are produced within a process chamber by introducing a process gas at vacuum pressure into the chamber and then coupling electrical energy into the chamber to create and sustain a plasma in the process gas. The plasma may exist at various ionization fractions from 10−6 up to a fully ionized plasma.
The plasma generally contains positively charged ions of working gas that are used for etching a surface of a substrate, sputtering material from a target for depositing a layer of the material onto such a substrate and ions of vaporized coating material to control the deposition of the material onto the substrate by ionized physical vapor deposition (iPVD). The plasma typically contains electrons equivalent in number to the positive charges in the plasma so that the plasma is macroscopically quasi-neutral.
Various ways of producing a plasma within a process chamber are used. Opposed electrodes can be oriented within the chamber to capacitively couple energy to the plasma. Microwave energy and electron cyclotron resonance (ECR) devices are also used. Inductive coupling of energy to the plasma is particularly desirable for producing a high-density plasma, particularly plasmas having a high-ionization fraction with a relatively low electron energy or plasma potential. Inductively coupled plasmas (ICP) often use a coil or antenna, shaped and positioned with respect to the processing chamber to inductively couple energy into the processing chamber and thus create and sustain a plasma therein.
In some ICP systems, an inductive coil or antenna is positioned around or proximate the top portion or another end of the chamber to create a plasma within the chamber. Such an antenna is positioned on one side of a dielectric plate or window in the wall of the processing chamber, and electromagnetic energy from the antenna is coupled through the dielectric window and into the plasma. One suitable dielectric material for a window or chamber sidewall is quartz.
The geometry of an ICP system is a factor in determining both plasma density and uniformity, which, in turn, can affect the processing uniformity over the area of the substrate. It has often been regarded as desirable to produce a uniform, high-density plasma over a significantly large area so that large substrate sizes can be accommodated. Ultra large-scale integrated (ULSI) circuits, for example, are presently formed on wafer substrates having diameters of 200 mm and 300 mm.
In conventional sputter coating systems of the prior art, it has been recognized that the geometry of sputtering targets affected the uniformity of coatings on wafers. In U.S. Pat. No. 4,957,605, for example, it was determined that increasing material supplied from an annular ring toward the outer periphery of the target improved uniformity of coating on the wafer and improved step coverage. This patent discussed processes performed at such pressures that left line-of-sight paths for sputtered material to travel from target to wafer.
When ionized physical vapor deposition systems (iPVD) were developed, such as in U.S. Pat. No. 6,080,287, ring-shaped targets were found advantageous. An RF source in the center of the target was provided to couple energy to form a secondary plasma in the chamber that ionized material sputtered from the target. The ionized coating material aids in the coating of features on wafers as such features have become smaller. It was further found that the use of higher pressures in the chambers had advantages, as in U.S. Pat. No. 6,287,435. Such higher pressures tended to thermalize sputtered material in the chamber and masked the effects of target geometry on coating uniformity.
In etching systems and processes, pressures are usually not as high as in the high pressure iPVD systems discussed above, but because no target geometry is available, etching systems that use ICP must rely on the shape of the plasma to provide coating uniformity. In coating and etching, as well as plasma cleaning and other plasma processing systems, there remains an ongoing need for increased uniformity in the processing of wafers.
The most common inductively coupled sources have antennas in the form of coils with planar, cylindrical or dome-shaped geometry. Other coil structure includes more complex shapes having combined (hybrid) or dual coil configurations, multiple small solenoids, multiple spirals, multizone ICP enhanced PVD, toroids, transmission lines, embedded coils, planar helicon (serpentine) antennas, and parallel conductor antennas. Three-dimensional coils, and deposition baffles and Faraday shields proposed to be used with them, have been described by the present applicant in U.S. Pat. Nos. 6,237,526 and 6,474,258, and in U.S. patent application Ser. Nos. 10/080,496 and 10/338,771.
The semiconductor manufacturing tools for 300 mm wafers are becoming of increased demand, which call for larger processing chambers, typically more than 450 mm in diameter. Such chamber enlargement is needed to reduce plasma losses within the processing zone by reducing the effect of the chamber walls on the bulk plasma, and to accommodate inside the chamber hardware such as shields, lamps, diagnostic devices, monitoring and control instrumentation, that are becoming increasingly required. For such chambers, the inductive elements are made larger as well, in a range of from 400 to 500 mm diameter for cylindrical or solenoid coils and up to 350 mm for spiral antennas used to inductively couple energy into the chambers for the processing of 300 mm wafers.
Increased coil size has required larger dielectric windows to allow RF energy to penetrate into plasma efficiently and to withstand atmospheric forces. Scaling up an external antenna for large area plasma in a conventional inductively coupled discharge meets such difficulties as requiring a thick dielectric window, a large inductance of antenna, and an enormous increase in power required to provide identical plasma conditions for etching or deposition. For example, inductance of the antenna is proportional to the square of the number of turns, and voltage drop across the antenna increases with number of turns. The voltage at the ends of such large scaled antennas can easily reach values above 10 kV at typical coil currents at the industrial frequency of 13.56 MHz. Such a high voltage is a hazard and results in an intense capacitive coupling between the antenna and the plasma, and an increased potential for sparking and arcing.
Etch uniformity at the wafer is given by ion flux towards the wafer and it is determined by plasma density distribution. Typically, an ICP source with a spiral coil produces a plasma distribution with a peak at its center. Use of solenoids with larger diameters is limited due to induced high voltages at such coils. The geometry of an inductively coupled plasma source, specifically of the antenna, is a significant factor in determining both the plasma and processing uniformity over a large area. With ICP sources, plasma is excited by heating electrons in the plasma region near the vacuum side of the dielectric window by oscillating inductive fields produced by the antenna and coupled through the dielectric window. Inductive currents, which heat the plasma electrons, are derived from RF magnetic fields produced by RF currents in antenna. The spatial distribution of the magnetic field is a function of the sum of the fields produced by each portion of the antenna conductor. The geometry of the inductive antenna can in large part determine spatial distribution of the plasma ion density within the reactor chamber.
In some cases, a shield that is transparent to the inductive component of the electromagnetic field is used to suppress the capacitive coupling from antenna to plasma and to prevent a conductive or contaminating layer from building up on the dielectric window. The geometry and structure of such a shield can have an effect on the spatial distribution of plasma inside the chamber as well.